1. Field of the Invention
The present invention relates to a sulfonium salt, a resist composition, and a patterning process.
2. Description of the Related Art
In recent years, as LSI advances toward a higher integration and a more rapid processing speed, finer pattern rules are being requested; and in this trend, a far ultraviolet lithography and a vacuum ultraviolet lithography are promising for the next generation fine patterning technologies. Especially, a photolithography using an ArF excimer laser beam as a light source is an indispensable technology for an ultrafine patterning process with the size of 0.13 μl or less.
The ArF lithography started to be used partially from manufacturing of a device with the 130-nm node, and then it became a main lithography technology from the 90-nm node device. As the lithography technology for the next 45-nm node device, a 157-nm lithography using a F2 laser was initially considered as a promising technology; however, a delay in development thereof due to several problems was indicated. Accordingly, an ArF immersion lithography emerged rapidly because by inserting a liquid whose refractive index is higher than air, such as water, ethylene glycol, and glycerin, between a projection lens and a wafer, the number of aperture (NA) of the projection lens therein can be designed to be 1.0 or more thereby attaining a high resolution (for example, see Non-Patent Document 1); and this is now in the stage of practical use. For this immersion lithography, a resist composition not readily eluting into water is being requested.
In the ArF lithography, in order to avoid deterioration of a precise and expensive optical material, a highly sensitive resist composition that can express a sufficient resolution with a small exposure dose is requested; and in order to realize this, the most generally used method is to select each ingredient having a high transparency at the wavelength of 193 nm. As to a base resin for example, polyacrylic acid and a derivative thereof, norbornene-maleic acid anhydride alternating copolymer, polynorbornene, a ring-opened metathesis polymer, a hydrogenated ring-opened metathesis polymer, and the like have been proposed; and these bring about a certain level of results in enhancing a transparency of a resin.
In recent years, a negative tone resist by an organic solvent development as well as a positive tone resist by an alkaline development has been receiving an attention. In order to resolve a very fine hole pattern, which cannot be achieved by a positive tone, by a negative tone exposure, a negative pattern is formed by an organic solvent development using a positive resist composition having a high resolution. In addition, a study to obtain a doubled resolution power by combining two developments of the alkaline development and the organic solvent development is going on.
As to the ArF resist composition for the negative tone development by an organic solvent, an existing positive ArF resist composition can be used; and patterning processes thereof are disclosed in Patent Document 1 to 3.
In order to catch up the rapid fine patterning trend in recent years, development of a resist material as well as a process technology is progressing day by day. Various photo-sensitive acid generators have been studied; and a sulfonium salt of a triphenylsulfonium cation and a perfluoroalkane sulfonate anion is generally used. However, a generated acid of a perfluoroalkane sulfonic acid, especially perfluorooctane sulfonic acid (PFOS), has concerns about low degradability, biological concentration, and toxicity. And therefore, its use in the resist composition becomes difficult, so that currently a photo-sensitive acid generator to generate perfluorobutane sulfonic acid is being used; however, when this generator is used in the resist composition, it is difficult to achieve a high resolution because of a high diffusibility of the acid generated therefrom.
To the above problems, various partially fluorine substituted alkanesulfonic acids and salts thereof have been developed, and a photo-sensitive acid generator having 2-acyloxy-1,1,3,3,3-pentafluoropropane-1-sulfonic acid has been developed in the points of easiness in introducing a substituent which controls solubility into a resist solvent, stability, diffusibility, etc. (see Patent Document 4). Meanwhile, in Patent Document 4, as a conventional technology, a photo-sensitive acid generator generating an α,α-difluoroalkanesulfonic acid by exposure, more specifically a photo-sensitive acid generator generating di(4-tert-butylphenyl)iodonium=1,1-difluoro-2-(1-naphthyl)ethanesulfonate or an α,α,β,β-tetrafluoroalkanesulfonic acid has been mentioned. However, although in both generators, the fluorine substitution rates are reduced, there are problems that they do not have a decomposable substituent such as an ester structure, etc., so that they are insufficient in the viewpoint of degradability. Further, it involves the problems that there is a limitation in the molecular design to change the size of the alkanesulfonic acid, and the fluorine-containing starting substance is expensive, etc.
Further, a triphenylsulfonium cation has been widely used as a cation, but there is a drawback that absorption at ArF exposure wavelength (193 nm) is large, so that transmittance of the resist film is lowered and resolution is low in some cases. Thus, a 4-alkoxy-1-naphthyltetrahydrothiophenium cation, etc., have been developed (see Patent Document 5) for the purpose of a high sensitivity and a high resolution, and a resist composition with a combination of a resin(s) having a plural number of acid-labile groups (see Patent Document 6) has been disclosed.
However, accompanying with narrowing of a line width of a circuit, influence of the contrast degradation by an acid diffusion becomes still more serious in the resist composition. It is the present status that the above-mentioned sulfonium salts cannot meet the required performance of the resist. This is because the pattern dimensions get close the diffusion length of the acid. Specifically, it causes degradation of mask reliability and pattern rectangularity, unevenness of the fine line pattern (line width roughness: LWR), etc.
Also, since the sulfonium salt type photo-sensitive acid generator is an ionic compound, the point that it is easily eluted to water in the ArF immersion lithography than the other resist composition is one of the problems, so that a molecular design that is difficultly elute to water as much as possible has been desired.